Lung volumes and functional residual capacity

Tidal volume (TV or V_T) is the air moved in an ordinary breath. Inspiratory reserve volume (IRV) is the extra air that can be inspired after an ordinary inspiration. Expiratory reserve volume (ERV) is the extra air that can be expired after an ordinary expiration. Residual volume (RV) remains after maximal expiration.

Capacities combine those volumes. Inspiratory capacity is TV + IRV. Vital capacity is IRV + TV + ERV. Total lung capacity is vital capacity + RV.

Functional residual capacity (FRC) is the air left in the lungs after a normal expiration. It includes ERV + RV. At relaxed, no-flow equilibrium, inward lung recoil balances the outward recoil of the chest wall.

There is a numerical difference in this note. The attached reference graph places FRC at 30 mL/kg, while the typed answer and textbook give approximately 45 mL/kg. The graph’s scale illustrates the relationships; its species and measurement conditions were not supplied. It does not confirm the 45 mL/kg estimate. Neither value is a universal canine or feline reference interval. Body position, sedation or anaesthesia, and the measurement conditions can change FRC.

Residual gas cannot be measured from an exhaled-volume trace alone. Lung-volume methods include gas dilution or washout and body plethysmography. The important distinction here is the volume remaining after an ordinary expiration versus the smaller residual volume after maximal expiration.

References

Johnston & Tobias, Veterinary Surgery: Small Animal, 2nd ed., Ch. 105; Larsson et al. 1992; Rozanski et al. 2010; ERS/ATS lung-volume technical statement 2023 (definitions and methods).